1. Field of the Invention
This invention relates to an audio transducer, and more particularly, to a speaker structure capable of operating as a full range unit.
2. Description of the Prior Art
A true full range speaker is one which utilizes a single driver to reproduce the whole audible frequency range, as opposed to an arrangement in which two or more drivers, such as woofers and tweeters together, with the required crossovers, are used to produce such a range. In the past, manufacturers have produced speakers that have been termed full range units, but these units have suffered from various problems, and are usually only wide range, not full range. Usually the known units have poor bass output, or if the unit is large enough to have good bass characteristics, they have problems with high frequencies, either because of the high mass of the large driver, or because the large radiating area results in a narrow treble dispersion.
In most known structures of speakers, the diaphragm is driven as a piston. Such a pistonic radiator will produce a substantially planar wavefront when the wavelength of the frequency being propogated is less than the dimensions of the diaphragm. Although numerous designs, which utilize a thin flexible membrane as the diaphragm, have been developed, they have, in the main, made provision for driving the diaphragm as a piston and have therefore experienced the same type of disadvantages. Examples of such pistonic magnetic type speakers are shown in U.S. Pat. No. 3,919,499, granted Nov. 11, 1975, to Winey and U.S. Pat. No. 4,020,296, granted Apr. 26, 1977, to Dahlquist. In these patents, it can be seen that the conductors are spread over a wide area of the diaphragm which gives the effect of the diaphragm being driven as a piston. Dipole or planar units, i.e., units which have no box or cabinet for bass loading, but consist of a thin diaphragm stretched on a frame, may be open on both sides and therefore radiate equally in both directions. Such a design has many desirable characteristics, and although such a design is not a recent developement, it is slow in its commercial progress because of cost.
As indicated, most known planar designs follow conventional technology and attempt to drive the moving element as a piston. Full-range pistonic units have been designed. The listener must sit directly "on axis" to consistently hear all frequencies from such transducers. If such a full range speaker achieves a linear frequency response, it then tends to have an energy spectrum which is percieved to rise in intensity with frequency. This is because the radiated sound is increasingly concentrated on axis, as treble beaming from the large diaphragm comes into effect. This must then be counteracted by other means, such as a mass induced high frequency roll off. Although this can linearize the frequency response it does not help treble dispersion.
A more common response to these problems is to make use of multiple drivers. Crossovers are then required to separate the appropriate frequency bands and direct them to the different drivers. Each driver has suitable mass and dimensions for reproducing those frequencies without compromising directionality. Even though planar designs can be built with inherently low mass, and consequent predictable out-of-band behavior, the same kinds of crossover filters are still generally employed, introducing phase errors in the reproduced signal. Furthermore, each specialized driver is located at a different point in space. This causes path differences between diaphragm and ear, producing additional phase/frequency anomalies. In some cases, these various factors can to some extent complement each other, but it remains a complex and inexact task to achieve proper sonic integration between the multiple drivers of any two or three way system.
Speakers are being produced which attempt to circumvent these problems by avoiding having the diaphragm act as a piston. In such a speaker, the diaphragm is driven in sections. An appropriate time delay is applied to each section, so as to achieve a peaked wavefront at all frequencies, instead of an increasingly planar wave with rising frequencies. Thus, the speaker acts as a small virtual source. One example uses annular concentric rings to reconstruct a virtual point source while another uses vertical sections to reconstruct a virtual line source, such an arrangement being described in more detail below. In all such cases, the design drives the entire diaphragm area and requires complex delay electronics to achieve the required wave shaping. In many cases the effect is also enhanced by reducing the high frequency content of the signal passed to the outer rings/sections of the driven area, so that the high frequency radiator is actually substantially smaller than the whole diaphragm. Although the performance of these speakers can be impressive, the complexity of the design and the extensive electronics involved makes such structures expensive.
In a development made by Peter J. Walker and incorporated in products sold under the trademark QUAD, an electrostatic transducer has some features which at first appear similar to the present invention. Walker uses electrical segmentation of a single panel to achieve a time delayed wavefront which stimulates a virtual point source located behind the plane of the dipole panel. This is done by dividing the signal bearing stators on either side of the charged diaphragm into annular rings, each ring being fed via a delay line. Only the central section is fed real time signal, the rest being increasing delayed with distance from the center so that the same kind of peaked wavefront as the present point source embodiment is achieved. Although the end result of Walker's development is in some way similar, it is clear that the means by which the effect is achieved is completely different. The entire diaphragm of walker is driven, and the delay is achieved electrically, through a complex series of filters. Not only is a delay applied, but some degree of frequency shading also occurs, that is to say, the outer annular rings are not driven full range, but inevitably feature a high frequency roll off, due to the inductive/capacitive nature of the delay lines. This is actually beneficial in this case because the central area, which is driven full range, is rather larger than his ideal, and tends to beam output in the highest audible octave. Thus, the speaker is neither truly full range, nor without crossover filters. However, it is the only dipole transducer, of which applicant is aware, which really focuses on creating the same type of wavefront profile as the present invention.